In memory design, a sense amplifier is a critical analog circuit. In EEPROM and flash memories the sense amplifier function is twofold. First, the precharge of a bitline to a proper value of clamp voltage is significant for providing calibrated sensing of a selected memory cell. A sense amplifier performs a critical function in assuring the bitline is precharged to a clamp voltage of sufficient magnitude to ensure proper sensing of memory cell current. Second, sensing a selected memory cell current flowing through the bitline is also critical. Magnitude of the memory cell current depends on the state of the cell and must be determined promptly and accurately in order to sustain a high level of system performance.
With reference to FIG. 1, a prior art sense amplifier 100 is based on a differential structure 130 in order to compare a current coming from a selected memory cell (not shown) to a fiducial current coming from a reference cell (not shown). The fiducial current flows on a reference bitline 110 and a memory cell current flows on a bitline 120 from the selected memory cell. The differential structure 130 compares the relative magnitudes of the voltages on the reference bitline 110 and the bitline 120. As different memory cells with differing states (i.e., cells storing either a 1 or a 0) are selected, the current flowing on the bitline 120 varies, producing different voltage levels at the corresponding input to the differential structure 130. The differential structure 130 amplifies the voltage difference and provides a corresponding voltage level to an output amplifier 140. The output amplifier 140 in turn amplifies and inverts the voltage level and provides it to the sense amplifier output terminal OUT.
With reference to FIG. 2, a prior art single-ended current sense amplifier 200 has a precharge circuit 205 to maintain a stable voltage on a bitline BITLINE, a sensing circuit 210 coupled to the bitline BITLINE for sensing an amount of current flowing into the bitline BITLINE, a direct current amplification circuit 215 coupled to the sensing circuit for amplifying the current sensed on the bitline BITLINE, a current-to-voltage conversion circuit 220 for converting the sensed current to a voltage, and a voltage amplification circuit 225 for amplifying the voltage at the sense amplifier output. The sense amplifier also includes an overshoot filtering circuit 230 to filter out positive glitches on the bitline BITLINE.
Generally, a single-ended structure provides an advantage by eliminating a need to have a reference cell and a comparator circuit as are commonly used in differential sense amplifier structures. The single-ended amplifier provides a savings in test time and in the amount of die area used by the sense amplifier circuit. Additionally, the single-ended structure provides other advantages over the standard differential structures such as providing less sensitivity to mismatched circuit components, less process variation sensitivity, and improved access time at low supply voltages.